Capacitance gauges have been used in environments such as air craft, for measuring the level of fuel in a wing tank. Capacitance gauges have no moving parts and are therefore very reliable, especially in hostile environments were vibration and temperature extremes render mechanical level sensors useless. Capacitance gauges allow for relatively simple compensation of various tank shapes, where linear changes in fluid level do not correspond to linear changes in fluid volume.
Capacitance level sensors comprise two plates which establish a capacitor. All capacitance level sensors are based on the fact that electrical capacitance between two electrodes or plates is described by: EQU C(pF)=8.85.times.10.sup.-2 S e (N-1)/d
where:
S =area of one plate in cm.sup.2
N =number of plate
d =distance between plates (cm)
e =dielectric constant
Changes in dielectric constant in the medium separating the plates of the measuring capacitor will cause a change in measured capacity. Air has a nominal dielectric constant equal to 1.0, and common oils or fluids such as kerosine or gasoline having nominal dielectric constant of 2.0. Such fluids rising between two parallel plates will increase the net capacitance of the measuring cell as a function of fluid height. The fluid being measured may not vary in dielectric constant, or changes in measured capacitance may erroneously be attributed to level changes.
Conventional capacitance level gauges can not handle fluids of varying dielectric constant. Other fluids such as alcohol and water, which may be present in varying amounts in modern automotive fuels are not compatible with current capacitance gauges and act as a "poison" to the gasoline by artificially increasing the bulk dielectric constant of the mixture by disproportionate amount. Ethanol and methanol have dielectric constants of 24 and 31, respectively, and water has a dielectric constant of 78. Relatively small concentrations of these additives to gasoline will change the dielectric constant of the resulting mixture by a great amount. In most cases, the addition of 10% to 15% ethanol to gasoline will raise the effective dielectric constant to approximately 5.3.
U.S. Pat. No. 4,470,300, issued Sept. 11, 1984 in the name of Kobayashi discloses a level gauge system for determining the alcohol concentration in gasohol. The patent discloses a level gauge which uses a sensing capacitor connected as an RC time constant to an astable miltivibrator for determining the level. The frequency of the output from the astable multivibrator indicates the capacitances of the sensor. The astable multivibrator is mounted on the top of the fuel tank and is connected to the capacitor so as the generate a signal which oscillates at a frequency depending on the capacitance of the capacitor. A period detector is connected to the oscillator in order to detect the period of the oscillatory signal therefrom. The oscillator includes the RC time constant arranged so that the oscillator generates pulses and frequencies having a minimum value which is higher than the predetermined reference value depending on the alcohol concentration in gasohol and the kind of the alcohol in gasohol.
The addition of alcohol to gasoline is the primary mechanism responsible for changes in the combustion characteristics, therefore, the alcohol constant would be quite valuable. By continually monitoring the nature of fuel mixtures in the tank, the engine control computer could program the engine operation for optimal performance in the minimum emission for any given fuel mix. A convenient way of determining the ratio of alcohol in gasoline is by monitoring the effective dielectric constant of the gasohol mixture. Incorporating a reference or normalizing capacitance cell into the level sensing capacitance gauge achieves compensation for changes in dielectric constants due to temperature variation and mixture.
U.S. Pat. No. 4,590,575, issued May 20, 1986 in the name of Emplit discloses a time base system for determining the level of fluid which utilizes a reference capacitance sensor and level capacitance sensor. The system is an on-line system wherein the time intervals from multivibrators determines the capacitance. The system has a measurement probe whose capacitance is a function of the level of substance in the tank and a reference probe whose capacitance is a function of the dielectric constant of the substance. Each probe is coupled to a multivibrator whose output frequencies are a function of the capacitance between the respective probe and the tank. The output signals of the two multivibrators circuits are coupled to logic and switching circuits. The counter counts the number of input pulses and when a predetermined number have been counted, the counter inverts the binary state of the output and changes which sensor probe is transmitting the signal. The output from the transmitter includes pulses whose time duration varies as a function of probe capacitance. The low level pulse segments correspond to the frequency of the reference multivibrator and the high level pulses represent the output of the frequency of the level multivibrator. When the output signal is received, the signal is separated into two time intervals signals, one representing each probe. The duration of the segments used by the counter produced for the microprocessor and input representatives of the capacitance of the measurements and thereby the level of substance in the tank. The problem with such a system is that the sizes of the tank must be known since capacitance is established between a probe and a tank. Furthermore, the signal produced alternates between the reference signal and the measurement signal with time delays therebetween resulting in a less accurate system and slower responding system.
None of the prior art accurately produces a signal which simultaneously comprises information of level and dielectric constant, or compensated level measurement for any shape of container.